I have a 15ft Catamaran with a 40hp evinrude, Should I go bigger on the engine size?

Question

The boat is quite big for 15ft.

it's 6ft wide an only drafts 5in in the back fully loaded + me.

i got this 40hp evinrude outboard an mounted it an tuned the engine an got the right prop for it an it gets GREAT hole shots

an tops off at 42MPH *GPS'ed* an is getting about 6miles per gal which is amazing for a 2 cycle.

But my question is i have a opportunity to give this older man my 40hp engine

for his 70 hp 2 stroke yamaha.

it's a 2004 an low miles.

* he totaled his bigger boat an got a smaller one with no engine an still has his 70hp with him *

So I was wondering if it would be smart to upgrade to a higher

hp engine an would it still give me that good gas mileage like my 40hp?

Because someone told me that if I have a bigger engine that means I can have more pitch in my prop an that will make it to where I can run the bigger engine at a lower RPM which means better MPG.

So that would be smart..right?

Answer 1

Yes and No....you have to keep in mind the power-weight ratio. The 40 horse evinrude weighs 240 lbs, while the 70 horse Yamaha weighs 235..which for you is excellent. You are getting alot more horsepower per lb of weight with the Yamaha, and your fuel burn will decrease as your cruise speed will increase to about 45 mph with your current prop. Remember, these engines cruise at around 3500 to 4000 rpm, anything more or less will increase fuel burn. I would say go for it.